The present invention relates generally to a solar cell power panel having excellent weather-proof characteristics.
A solar cell is a type of photoelectric transducer which converts energy of a light quantum incident on a P - N junction of a semiconductor into an electric current, the fundamental construction of which is fully disclosed in U.S. Pat. No. 2,750,765 of D. M. Chapin et al issued on Feb. 5, 1957.
In the past, one or more solar cells contained within a solar cell assembly were housed in conventional plastic packages which were made of, for example, an acrylic or polycarbonate resin. Since a solar cell power system containing a multiplicity of these solar cell assemblies is usually placed outdoors, the plastic packages would be denaturalized, deteriorated or become opaque due to ultraviolet radiation, deleterious gas and variations in atmospheric temperature. This resulted in a reduction in the operating life of the solar cell power system.
In particular, due to ultraviolet radiation, there was observed a remarkable deterioration in the plastic packages and fillers provided for embedding and potting the solar cells within the packages. As a consequence, the packages became yellowed with an accompanying reduction in the amount of transmitting light, and the package sealing means were damaged which allowed the invasion of moisture. Therefore, there is still a great need to develop a new packaging material which is stable to ultraviolet radiation and capable of providing complete protection for the solar cells.
Moreover, since the solar cell power system is exposed to the direct sun, an increase in the interior temperature thereof is produced. This causes temperature differences with respect to the temperature of the system which is cooled during the night, thereby producing deterioration in the packaging material and thus providing one of the important factors affecting reduction in the photoelectric energy conversion efficiency. For example, it has been reported that when the temperature of a solar cell power system is increased 20.degree. C., reduction of output of around 10% is noted. It thus is required that the solar cell power system be free from any temperature increase due to daylight radiation.
In accordance with one of the typical prior art assemblies, several solar cells were mounted on a printed circuit board opaque to or non-transmissive to light, and then enclosed within a plastic package of which at least the light receiving surface was transparent. When the rear portion of the printed circuit board carrying no solar cells therein is exposed to light radiation, that portion is heated by absorption of the incident light.
It is, therefore, an object of the present invention to provide an improved solar cell assembly wherein one or more solar cells are sandwiched between a pair of glass laminations or glass sheets through the use of a light-transmissive resin adhesive.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. Advantageously, the glass sheets are readily available and manifest excellent physical and chemical stability for ultraviolet radiation, deleterious gas, variations in temperature, etc. In addition, they require no complicated maintenance procedure such as surface cleaning.
As discussed previously, one or more solar cells are sandwiched between the two glass sheets. When a plurality of the solar cells are housed in a single package, they are electrically connected in series or parallel with each other through the use of lead wires or lead frames. Instead of these interconnection means, electrically conductive material, e.g., metal, may be disposed in a predetermined pattern on one of the glass sheets and then the solar cells may be secured thereon by soldering technique. Each solar cell may be provided at its rear surface with + and - electrodes.
A filler variable for embedding and potting the solar cells between the two glass sheets may be of the room temperature vulcanizing type, the thermo-setting adhesive type or the thermoplastic adhesive type. In the aforementioned methods, a spacer is first tightly disposed between the peripheral portions of the two glass sheets and the cavity surrounded by the glass sheets and the spacer is filled with the above-discussed adhesive.
The adhesive filler preferably has the following characteristics:
(i) Transparency, in particular, the filler should be transparent to light in the range from 4000 to 11,000 A (that is, the sensitivity region of a Si solar cell). PA1 (ii) Excellent weather-proof characteristic. In particular, no discoloaration should be observed due to ultraviolet radiation. PA1 (iii) No bubbles. Bubbles should be readily removable. PA1 (iv) Rubber-like resiliency. Little or no transmission of vibration and shock is observed. PA1 (v) No by-products which attack the electrodes of the solar cells. PA1 (vi) No difference in the thermal-expansion coefficient from that of the spacer material. PA1 (i) Tight adhesion with glass. PA1 (ii) Good weather-proof characteristics, that is, its physical characteristics are not harmed when exposed to the ambient environment. PA1 (iii) No by-product which chemically attacks the solar cells and particularly the electrodes thereof. Generation of sulfur gas sould be prevented. PA1 (iv) Rubber-like resiliency over a wide range of temperature. PA1 (v) No difference of thermal-expansion coefficient from that of the above-discussed filler.
A silicone resin is one of the most favorable materials for providing the above-listed conditions and, in particular, exhibits excellent weather-proof characteristics. In addition, the following condition is necessary in order that undesired stress does not act on the package due to differences in temprature.
Also, the spacer material should fulfill the following conditions;
Metal, plastics, rubber, etc., are considered as proper materials. Metal materials are most favorable with respect to foregoing condition (ii) but not with respect to conditions (iv) and (v). More particularly, the preferred spacer materials for meeting the totality of these conditions are resinous materials of which the composition is substantially identical with that of the filler. Preferably, the spacer materials should have good adhesion to glass, as compred with the filler material. The spacer materials need not be transparent, as distinguished from the filler material.
As noted earlier, the filler may be of the thermoplastic (hot melt) adhesive type. Two glass sheets adhered through such type of the filler (generally called "laminate glass") are now available in a wide range of applications, for example, the window glass for airplanes and the front glass of automobiles because of its inherent advantages of remarkedly good shock-proof, heat-proof, water-proof characteristics. Further, adhesion to resin and glass is tight and softening thereof is expected at temperatures below the melting point of solder. Nevertheless, this type of filler is somewhat poor with respect to resiliency, which increases the tendency to transmit shock to the solar cells embedded therein.